fix(server): prefer pwsh as the default Windows shell

[adammansfield]

What Changed

Prefer pwsh.exe as the default Windows shell, and do not rely on ComSpec to allow user shell selection on Windows.

Why

Do not rely on overriding ComSpec to allow user shell selection on Windows because many tools require it to be cmd.exe. For example, setting ComSpec as pwsh.exe and launching T3 Code breaks the "Open" Visual Studio Code button as it no longer does anything when clicked.

Prefer pwsh.exe because it matches Visual Studio Code's default shell behavior on Windows when pwsh.exe (PowerShell 7) is available.

UI Changes

No UI changes.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes -- no UI changes
• I included a video for animation/interaction changes -- no animation/interaction changes

Validation

• bun fmt
• bun lint
• bun typecheck
• cd apps/server && bun run test -- src/terminal/Layers/Manager.test.ts -t "retries with fallback shells when preferred shell spawn fails"

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Note

Medium Risk
Changes Windows terminal spawn defaults and fallback order, which can affect startup behavior on systems without PowerShell 7 or with unusual shell setups; non-Windows behavior is unchanged.

Overview
Windows terminal sessions now prefer pwsh.exe by default. defaultShellResolver no longer uses ComSpec/cmd.exe, and the Windows fallback list is reordered to try pwsh.exe then powershell.exe before falling back to ComSpec and cmd.exe.

Tests were updated to reflect the new Windows behavior, including allowing pwsh.exe as a valid fallback and accounting for Windows command normalization differences.

^{Reviewed by Cursor Bugbot for commit 4bc221f. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

[!NOTE]

Prefer pwsh.exe as the default Windows shell in terminal manager

• Changes defaultShellResolver to return pwsh.exe unconditionally on Windows, replacing the previous default of ComSpec/cmd.exe.
• Updates resolveShellCandidates fallback order on Windows to [requested, 'pwsh.exe', 'powershell.exe', ComSpec, 'cmd.exe'], prioritizing pwsh.exe ahead of cmd.exe.
• Behavioral Change: Windows users will now get a pwsh.exe session by default instead of cmd.exe; systems without PowerShell Core installed will fall back to powershell.exe then cmd.exe.

^{Macroscope summarized 4bc221f.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR changes the default Windows shell from cmd.exe to pwsh.exe (PowerShell Core), which is a runtime behavior change affecting all Windows users. While fallback logic exists, changing default user-facing behavior warrants human review.

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